Linear actuators are basic elements of various kinds of machines, mechanical devices and they are widely used also in the field of automation. A guided linear movement is required in an actuator when in spite of the number and kind of forces acting thereon only a linear displacement can take place. A large number of linear guiding mechanics is known in which a precisely tooled profiled guiding member defines a liner path along which a moving element can be displaced by direct sliding or rolling by means of a ball bearing. The guiding member should be as rigid as possible and there should be a minimum clearance between the moving part and the path in transversal direction.
For that reason the guiding elements are made generally of steel or of a similarly firm material on which the guiding path is tooled with a high degree of precision. This justifies the high price of precise linear guiding mechanisms.
Such guiding mechanisms can be driven by linear actuators e.g. by hydraulic or pneumatic cylinders. The connection between the linear guiding mechanism and the actuator should provide a certain degree of lateral play to be able to compensate any angular difference or lateral displacement that can take place between the axes of the guiding mechanism and of the actuator, respectively.
In the European laid open publication No. EP 0 134 398 of Mar. 20, 1985 a linear actuator is described in which the interior of the guiding member forms the cylinder of the actuator and there is a piston driven in the cylinder which is connected to a moving member rolling on a pair of opposing outer guiding surfaces of the guiding member by means of ball bearing. In such a combined design the mechanical length of the assembly is reduced compared to the separate implementation of the actuator including the cylinder and of the linear guiding mechanism, however, a precision guiding effect is obtained only if the guiding member that houses the cylinder is manufactured by a high degree of precision. If the guiding path and the axis of the cylinder are not precisely parallel, mechanical tensions can arise during the displacement and the design shown in that publication does not seem to have provided any means that could overcome such problems.
In the German laid open publication No. DE 35 13 214 a similarly combined actuator-cylinder design has been suggested, in which both the guiding member and the moving part have uniform cross sections along their entire lengths. It has also been suggested that these members be made of aluminum or even of a plastic material which are both light and soft. Owing to the uniformity of the cross sections and to the mechanical properties of the chosen material such members can be manufactured by cheap extrusion technique. Since the guiding surface of a linear guide should resist to wear, in this German publication the use of steel bars has been suggested which bars are abutting the profiled support surfaces of the members with uniform cross sections.
Although the use of cheap technique for the production of the main elements of the assembly could substantially reduce the costs, there have remained a number of problems connected with inherent properties of such technique. A first problem lies in that there is an inevitable dimensional variation between different extruded pieces which substantially exceeds the tolerance range acceptable for linear guides.
A second problem is connected with the mechanical loadability i.e. the resistance against mechanical effects that have force and torque components which are not parallel to the guiding path. These effects can impede smooth sliding of the moving element along the guided path or, alternatively a decreased loadability can substantially limit the field of use of such an actuator. The application of materials which are far less strong than steel seems to decrease mechanical strength compared to precisely tooled steel linear guiding mechanisms.